In recent years, along with enhancement of the operation rate of personal computers, and popularization of network infrastructure, as well as an increase in capacity of data storage and a decrease in its cost, occasions have increasingly occurred in which pieces of information such as documents and images, which have been provided in the form of paper printed matter, are received as simpler electronic information and viewed as received electronic information.
As viewing means for such electronic information, mainly employed are those of light emitting types such as conventional liquid crystal displays and CRTs or recent organic electroluminescence displays. Specifically, when electronic information includes document information, it is required to watch any of the above viewing means for a relatively long period. However, it is hardly stated that the above viewing means are human friendly. It is a common knowledge that light emitting type displays result in problems such as eye fatigue due to flicker, inconvenient portability, limitations in reading posture, necessity to look at still images, or an increase in power consumption when viewed for a long time.
As means to overcome the above drawbacks, known are reflection type displays (having memory function) which utilize outside light and consume no power to maintain images. However, it is difficult to state that due to the following reasons, they exhibit sufficient performance.
Namely, a system employing polarizing plates, such as a reflection type liquid crystal, results in a problem for a white display due to a low reflectance of approximately 40%. In addition, it is difficult to state that most methods to produce structuring members are simple and easy. Further, polymer dispersion type liquid crystals require high voltage and the contrast of the resulting images is insufficient because the difference in refractive indices between organic compounds is utilized. Still further, polymer network type liquid crystals result in problems such as application of high voltage and requirement of complicated TFT circuitry to enhance memory capability. Yet further, display devices employing electrophoresis require high voltage of at least 10 V and tend to suffer insufficient durability due to aggregation of electrophoretic particles.
On the other hand, electrochromic display devices, though being drivable at a low voltage of at most 3 V, result in insufficient color quality of black and common colors (namely yellow, magenta, cyan, blue, and red) and tend to result in problems such that, in order to secure memory capability, the display cell requires a complicated film structure such as vapor evaporation film.
As a method to overcome these problems of the electrochromic display technique, disclosed is, for example, an electrochromic display device employing an electrochromic display device which contains, between two electroconductive substrates at least one of which is transparent, a porous layer and an electrolyte containing an electrochromic dye which can be reversibly colored or decolored by means of at least one of an oxidizing reaction and an reducing reaction. It is stated that this electrochromic display device has a simple structure, the image is bright and easy to observe and that the electrochromic display device is capable of reducing power consumption (for example, refer to Patent Documents 1 and 2).
As the result of the intensive study by the present inventor, a new problem was found that, when the full color electrochromic display device produced according to the method of the abovementioned Patent Documents 1 or 2 was evaluated in terms of a memory property (in which the change of color hue with time was observed for the display device after a voltage was applied to the device and then the circuit was opened), color drift was observed with time, since the memory properties of each electrochromic dye forming the full color display were different.
Patent Document 1WO 2004/068231 pamphletPatent Document 2WO 2004/067673 pamphlet